I just took the info from the tecnical spec on the Westbourne models web site.
"Graupner 700 BB Turbo 12v
Ballraced motor with internal fan. Suitable for Power Gear 2:1 Or Direct drive.
Specification
Nominal voltage 12 V
Operating voltage range 7,2 ... 19,2 V
No-load rpm 11600 min–1
No-load current drain 2 A
Current drain at max. efficiency 12,5 A
Current drain when stalled 43 A
Max. efficiency without gearbox 75 %
Length of case, excl. shaft 67 mm
Diameter 44 mm
Free shaft length 14 mm
Shaft diameter 5 mm
Weight 350 g"
also found this
http://www.modelboatmayhem.co.uk/Modellers/Steve%20Beasley/Najade.htm which gives figures of 30 mins of 2 6V 7amp Lead acids
The cheapest way of getting more running time is to get some more sets of lead acid battries and change them over. We only get 15 to 20 mins with the Severn running flat out and thats on 18V 12A